An opening of a sac-like container called a “pouch”, and an opening of a cup-shaped container closed by a sheet-like lid are sealed planewise by a heat sealing method. The opening of those containers is sealed after filling the content therein, therefore, intrusion of the contents into the sealed portion may cause a sealing defect. In order to avoid such disadvantage, the sealing defects of this kind have been detected by various methods. For example, Japanese Patent Laid-Open No. 06-144416 discloses a non-destructive and total inspecting method of heat-sealed condition using infrared radiation. According to the teachings of Japanese Patent Laid-Open No. 06-144416, the temperature at each point along the seal-width direction of the heat-sealed part is detected by an infrared ray radiation superficial thermometer without contacting with the container. In case the detected temperature distribution is different from a previously determined appropriate temperature distribution of a non-defective product more than a predetermined value, the sealed portion is judged as an unacceptable sealing.
Meanwhile, Japanese Patent Laid-Open No. 2000-79917 discloses a method of inspecting improper seal. According to the teachings of Japanese Patent Laid-Open No. 2000-79917, a temperature distribution at the seal portion just after the heat seal operation is measured by processing thermal image information taken by an infrared-ray camera. The thermal information thus measured is compared with the thermal information about the seal portion sealed properly to judge propriety of the seal portion.